In lightweight construction, in particular in aircraft construction, use is increasingly being made of composite components made of fiber-reinforced plastics, which can withstand extreme mechanical loads and at the same time offer a high weight-saving potential. These components are formed with reinforcing fibers which are subsequently saturated or impregnated with a curable polymer material, for example a polyester resin, an epoxy resin or the like, to form the finished component.
The alignment of the reinforcing fibers in a component of this type has a decisive influence on its rigidity and strength. To achieve optimum mechanical properties, the reinforcing fibers should, if possible, follow the direction of loading and not have any undulations. In addition, it is desirable for each individual reinforcing fiber to be subjected to uniform loading.
With conventional semifinished products, such as, for example, woven or laid fiber fabrics, for reinforcement of the polymer material, not all conceivable fiber orientations can be realized, since there the reinforcing fibers always run with a specific orientation.
One possible way of complying with a requirement for fiber alignment in accordance with loading is the TFP process (“Tailored Fiber Placement”). This involves the laying of fiber strands for mechanical reinforcement (“rovings”), which are in turn formed by a multiplicity of discrete reinforcing fibers running parallel to one another, along any desired path curve and attaching them with the aid of a fixing thread on a backing layer, whereby the alignment of the individual fiber strands can be adapted virtually optimally to the flux of force acting on the finished composite component. The optimum utilization of the mechanical load-bearing capacity of the fiber strands that is achieved in this way can minimize their number, and consequently also the weight. Moreover, the cross section of the component can be adapted in an ideal way to the respective local loads. Furthermore, reinforcements can be formed specifically in zones that are subjected to particular loading, such as, for example, regions where force is introduced or the like, by laying additional fiber strands. The discrete reinforcing fibers are, for example, glass fibers, carbon fibers, aramid fibers or the like.
The production of fiber preforms by means of the TFP process is performed on customary CNC-controlled automatic sewing and embroidering machines, which are also used, for example, in the textile industry. Once all the required layers have been laid with fiber strands, the finished fiber preform, which generally already has the desired final contour, is placed in a closable mould, and impregnated with a curable polymer material and subsequently cured to form the finished composite component. A number of TFP fiber preforms and/or layers of reinforcing fabrics may be combined with one another here. The impregnation of the fiber preforms with the curable polymer material may be performed, for example, by means of known RTM processes (“Resin Transfer Moulding”) in a correspondingly designed mould. If appropriate, any backing layer material that protrudes beyond a given edge contour of the fiber preform is cut off before carrying out the RTM process.
However, with the fixing thread and the backing layer, the TFP process introduces into the fiber preform two components that no longer perform any function in the later composite component. Specifically, the backing layer causes difficulties in realizing an ideal sequence of layers and represents a not insignificant proportion of the overall weight, in particular if a number of fiber preforms are placed one on top of the other. Although the backing layer may also be formed by a woven reinforcing fabric, for example by a woven glass- or carbon-fiber fabric, even in this case at least some of the reinforcing fibers have an alignment that is not in accordance with the loading. Moreover, the woven reinforcing fabric is also impaired by the penetration with the sewing needle during the TFT process, so that the characteristic material values may be impaired.
Furthermore, it is known to use a solid foam core as a supporting structure for the forming of three-dimensional fiber preforms, the fixing threads being firmly clamped in the upper side of the foam core by means of a tufting method, so that there is no need for an additional lower fixing thread. In the case of this method, however, a special foam core must be kept in stock for each fiber preform with a different surface geometry.